Method and apparatus to reduce arcing in electrodeless lamps

ABSTRACT

A lamp and methods of forming are shown. In one example, a dielectric layer is formed over a gap between conductors in a plasma lamp. Electric arcing is reduced or eliminated, thus allowing tighter gaps and/or higher voltages. In one example a glass frit method is used to apply the dielectric layer. A lamp is shown with a barrier layer that prevents tarnish such as tarnish from sulfur exposure. The barrier layer reduces or prevents degradation of the lamp due to conversion of a conductor material to non-conductive tarnish material.

RELATED PATENT DOCUMENTS

This patent application is a continuation of and claims the benefit ofpriority under 35 U.S.C. §120 to U.S. patent application Ser. No.12/178,433, filed on Jul. 23, 2008, and issued as U.S. Pat. No.8,063,565, which claims the benefit of priority of U.S. ProvisionalPatent Application Ser. No. 60/951,417, filed on Jul. 23, 2007, thecontents of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The field of the present subject matter relates to systems and methodsfor generating light, and more particularly to electrodeless plasmalamps.

BACKGROUND

Electrodeless plasma lamps may be used to provide point-like, bright,white light sources. Because electrodes are not used, they may havelonger useful lifetimes than other lamps. In an electrodeless plasmalamp, radio frequency power may be coupled into a fill in a bulb tocreate a light emitting plasma. However, as the fill is ignited and theplasma heats up, the load conditions of the lamp may change. This canimpact the startup procedure as well as the electronics used to drivethe lamp.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example, and not limitation, inthe figures of the accompanying drawings, in which like referencesindicate similar elements unless otherwise indicated. In the drawings:

FIG. 1A shows a cross-section and schematic view of a plasma lampaccording to an example embodiment;

FIG. 1B shows a perspective cross section view of a lamp body with acylindrical outer surface according to an example embodiment;

FIG. 1C shows a perspective cross section view of a lamp body with arectangular outer surface according to an alternative exampleembodiment;

FIG. 1D shows a cross-section and schematic views of a plasma lamp,according to an example embodiment, in which a bulb of the lamp isorientated to enhance an amount of collectable light;

FIG. 1E shows a perspective exploded view of a lamp body, according toan example embodiment, and a bulb positioned horizontally relative to anouter upper surface of the lamp body;

FIG. 1F shows another perspective exploded view of the lamp body of FIG.1D;

FIG. 1G shows conductive and non-conductive portions of the lamp body ofFIG. 1D;

FIG. 1H is a schematic diagram of an example lamp drive circuit coupledto the lamp shown in FIG. 1D;

FIGS. 1I and 1J show cross-section and schematic views of furtherexample embodiments of plasma lamps in which a bulb of the lamp isorientated to enhance an amount of collectable light;

FIG. 1K is a schematic diagram showing an example lamp and lamp drivecircuit according to an example embodiment;

FIG. 2 is a schematic view of a plasma lamp according to an exampleembodiment;

FIG. 3 is another schematic view of a plasma lamp according to anexample embodiment;

FIG. 4 is a flow chart of a method according to an example embodiment;and

FIG. 5 is another flow chart of a method according to an exampleembodiment.

DETAILED DESCRIPTION

The example embodiments shown in the drawings will be described hereinin detail but it is to be appreciated that the example embodiment areopen to various modifications and alternative constructions, As such,there is no intention to limit the examples to the particular formsdisclosed. On the contrary, it is intended that all modifications,equivalences and alternative constructions fall within the spirit andscope of the appended claims. In the figures and description, numeralsindicate the various features of example embodiments, like numeralsreferring to like or the same features throughout both the drawings anddescription.

FIG. 1A shows a cross-section and schematic view of a plasma lamp 100according to an example embodiment. In example embodiments, the plasmalamp 100 may have a lamp body 102 formed from one or more soliddielectric materials and a bulb 104 positioned adjacent to the lamp body102. The bulb 104 may contain a fill that is capable of forming a lightemitting plasma. A lamp drive circuit 106 may couple radio frequencypower into the lamp body 102 which, in turn, may be coupled into thefill in the bulb 104 to form the light emitting plasma. In exampleembodiments, the lamp body 102 forms a waveguide that may contain andguide the radio frequency power. In example embodiments, the radiofrequency power may be provided at or near a frequency that resonateswithin the lamp body 102.

In example embodiments, the lamp body 102 has a relative permittivitygreater than air. The frequency required to excite a particular resonantmode in the lamp body 102 may scale inversely to the square root of therelative permittivity (also referred to as the dielectric constant) ofthe lamp body 102. As a result, a higher relative permittivity mayresult in a smaller lamp body 102 required for a particular resonantmode at a given frequency of power. The shape and dimensions of the lampbody 102 may also affect the resonant frequency as described furtherbelow. In an example embodiment, the lamp body 102 is formed from solidalumina having a relative permittivity of about 9.2. In someembodiments, the dielectric material may have a relative permittivity inthe range of from 2 to 100 or any range subsumed therein, or an evenhigher relative permittivity. In some embodiments, the lamp body 102 mayinclude more than one such dielectric material resulting in an effectiverelative permittivity for the lamp body 102 within any of the rangesdescribed above. The lamp body 102 may be rectangular, cylindrical orany other shape as described further below.

In example embodiments, the outer surfaces of the lamp body 102 may becoated with an electrically conductive coating 108, such aselectroplating or a silver paint or other metallic paint which may befired onto the outer surface of the lamp body 102. The electricallyconductive coating 108 may be grounded to form a boundary condition forthe radio frequency power applied to the lamp body 102. The electricallyconductive coating 108 may help to contain the radio frequency power inthe lamp body 102. Regions of the lamp body 102 may remain uncoated toallow power to be transferred to and/or from the lamp body 102. Forexample, the bulb 104 may be positioned adjacent to an uncoated portionof the lamp body 102 to receive radio frequency power from the lamp body102.

In the example embodiment of FIG. 1A, an opening 110 is shown to extendthrough a thin region 112 of the lamp body 102. The surfaces 114 of thelamp body 102 in the opening 110 may be uncoated and at least a portionof the bulb 104 may be positioned in the opening 110 to receive powerfrom the lamp body 102. In example embodiments, the thickness H2 of thethin region 112 may range from 1 mm to 10 mm or any range subsumedtherein and may be less than the outside length and/or interior lengthof the bulb 104. One or both ends of the bulb 104 may protrude from theopening 110 and extend beyond the electrically conductive coating 108 onthe outer surface of the lamp body 102. Such positioning may help avoiddamage being done to the ends of the bulb 104 from the high intensityplasma formed adjacent to the region where power is coupled from thelamp body 102. In other example embodiments, all or a portion of thebulb 104 may be positioned in a cavity extending from an opening on theouter surface of the lamp body 102 and terminating in the lamp body 102.In other embodiments, the bulb 104 may be positioned adjacent to anuncoated outer surface of the lamp body 102 or in a shallow recessformed on the outer surface of the waveguide body. In some exampleembodiments, the bulb 104 may be positioned at or near an electric fieldmaxima for the resonant mode excited in the lamp body 102.

The bulb 104 may be quartz, sapphire, ceramic or other material and maybe cylindrical, pill shaped, spherical or other shape. In one exampleembodiment, the bulb 104 is cylindrical in the center and forms ahemisphere at each end. In one example, the outer length (from tip totip) is about 15 mm and the outer diameter (at the center) is about 5mm. In this example, the interior of the bulb 104 (which contains thefill) has an interior length of about 9 mm and an interior diameter (atthe center) of about 2 mm. The wall thickness is about 1.5 mm along thesides of the cylindrical portion and about 2.25 mm on one end and about3.75 mm on the other end. In other example embodiments, the bulb 104 mayhave an interior width or diameter in a range between about 2 and 30 mmor any range subsumed therein, a wall thickness in a range between about0.5 and 4 mm or any range subsumed therein, and an interior lengthbetween about 2 and 30 mm or any range subsumed therein. Thesedimensions are examples only and other embodiments may use bulbs havingdifferent dimensions.

In example embodiments, the bulb 104 contains a fill that forms a lightemitting plasma when radio frequency power is received from the lampbody 102. The fill may include a noble gas and a metal halide. Additivessuch as Mercury may also be used. An ignition enhancer may also be used.A small amount of an inert radioactive emitter such as Kr₈₅ may be usedfor this purpose. In other embodiments, different fills such as Sulfur,Selenium or Tellurium may also be used. In some examples, a metal halidesuch as Cesium Bromide may be added to stabilize a discharge of Sulfur,Selenium or Tellurium.

In some example embodiments, a high pressure fill is used to increasethe resistance of the gas at startup. This can be used to decrease theoverall startup time required to reach full brightness for steady stateoperation. In one example, a noble gas such as Neon, Argon, Krypton orXenon is provided at high pressures between 100 Torr to 3000 Torr or anyrange subsumed therein. Pressures less than or equal to 760 Torr may bedesired in some embodiments to facilitate filling the bulb 104 at orbelow atmospheric pressure. In some example embodiments, pressuresbetween 400 Torr and 600 Torr are used to enhance starting. Example highpressure fills may also include metal halide and Mercury which have arelatively low vapor pressure at room temperature. An ignition enhancersuch as Kr₈₅ may also be used. In a particular example, the fillincludes 1.608 mg Mercury, 0.1 mg Indium Bromide and about 200 nanoCurieof Kr₈₅. In this example, Argon or Krypton is provided at a pressure inthe range of about 100 Torr to 600 Torr, depending upon desired startupcharacteristics. Initial breakdown of the noble gas may more difficultat higher pressure, but the overall warm up time required for the fillto fully vaporize and reach peak brightness may be reduced. The abovepressures are measured at 22° C. (room temperature). It is understoodthat much higher pressures may be achieved at operating temperaturesafter the plasma is formed. These pressures and fills are examples onlyand other pressures and fills may be used in other embodiments.

A layer of material 116 may be placed between the bulb 104 and thedielectric material of lamp body 102. In example embodiments, the layerof material 116 may have a lower thermal conductivity than the lamp body102 and may be used to optimize thermal conductivity between the bulb104 and the lamp body 102. In an example embodiment, the layer 116 mayhave a thermal conductivity in the range of about 0.5 to 10watts/meter-Kelvin (W/mK) or any range subsumed therein. For example,alumina powder with 55% packing density (45% fractional porosity) andthermal conductivity in a range of about 1 to 2 watts/meter-Kelvin(W/mK) may be used. In some embodiments, a centrifuge may be used topack the alumina powder with high density. In an example embodiment, alayer of alumina powder is used with a thickness D5 within the range ofabout ⅛ mm to 1 mm or any range subsumed therein. Alternatively, a thinlayer of a ceramic-based adhesive or an admixture of such adhesives maybe used. Depending on the formulation, a wide range of thermalconductivities are available. In practice, once a layer composition isselected having a thermal conductivity close to the desired value,fine-tuning may be accomplished by altering the layer thickness. Someexample embodiments may not include a separate layer of material aroundthe bulb 104 and may provide a direct conductive path to the lamp body102. Alternatively, the bulb 104 may be separated from the lamp body 102by an air-gap (or other gas filled gap) or vacuum gap.

In some example embodiments, alumina powder or other material may alsobe packed into a recess 118 formed below the bulb 104. In the exampleshown in FIG. 1A, the alumina powder in the recess 118 is outside theboundaries of the waveguide formed by the electrically conductivematerial 108 on the surfaces of the lamp body 102. The material in therecess 118 provides structural support, reflects light from the bulb 104and provides thermal conduction. One or more heat sinks may also be usedaround the sides and/or along the bottom surface of the lamp body 102 tomanage temperature. Thermal modeling may be used to help select a lampconfiguration providing a high peak plasma temperature resulting in highbrightness, while remaining below the working temperature of the bulbmaterial. Example thermal modeling software includes the TAS softwarepackage available commercially from Harvard Thermal, Inc. of Harvard,Mass.

In example embodiments, lamp 100 has a drive probe 120 inserted into thelamp body 102 to provide radio frequency power to the lamp body 102. Inthe example of FIG. 1A, the lamp also has a feedback probe 122 insertedinto the lamp body 102 to sample power from the lamp body 102 andprovide it as feedback to the lamp drive circuit 106. In an exampleembodiment, the probes 120 and 122 may be brass rods glued into the lampbody 102 using silver paint. In other embodiments, a sheath or jacket ofceramic or other material may be used around the bulb 104, which maychange the coupling to the lamp body 102. In an example embodiment, aprinted circuit board (pcb) may be positioned transverse to the lampbody 102 for the drive electronics. The probes 120 and 122 may besoldered to the pcb and extend off the edge of the pcb into the lampbody 102 (parallel to the pcb and orthogonal to the lamp body 102). Inother embodiments, the probes may be orthogonal to the pcb or may beconnected to the lamp drive circuit through SMA connectors or otherconnectors. In an alternative embodiment, the probes may be provided bya pcb trace and portions of the pcb board containing the trace mayextend into the lamp body 102. Other radio frequency feeds may be usedin other embodiments, such as microstrip lines or fin line antennas.

In an example embodiment, the drive probe 120 is positioned closer tothe bulb 104 in the center of the lamp body 102 than the electricallyconductive material 108 around the outer circumference of the lamp body102. This positioning of the drive probe 120 may be used to improve thecoupling of power to the plasma in the bulb 104.

A lamp drive circuit 106 including a power supply, such as amplifier124, may be coupled to the drive probe 120 to provide the radiofrequency power. The amplifier 124 may be coupled to the drive probe 120through a matching network 126 to provide impedance matching. In anexample embodiment, the lamp drive circuit 106 is matched to the load(formed by the lamp body 102, bulb 104 and plasma) for the steady stateoperating conditions of the lamp. The lamp drive circuit 106 may bematched to the load at the drive probe 120 using the matching network126.

A high efficiency amplifier may have some unstable regions of operation.The amplifier 124 and phase shift imposed by the feedback loop of thelamp circuit 106 should be configured so that the amplifier 124 operatesin stable regions even as the load condition of the lamp 102 changes.The phase shift imposed by the feedback loop may be determined by thelength of the loop (including matching network 126) and any phase shiftimposed by circuit elements such as a phase shifter 130. At initialstartup before the noble gas in the bulb 104 is ignited, the load mayappear to the amplifier 124 as an open circuit. The load characteristicsmay change as the noble gas ignites, the fill vaporizes and the plasmaheats up to steady state operating conditions. In various exampleembodiments, the amplifier and feedback loop are designed so theamplifier will operate within stable regions across the load conditionsthat may be presented by the lamp body 102, bulb 104 and plasma. Theamplifier 124 may include impedance matching elements such as resistive,capacitive and inductive circuit elements in series and/or in parallel.Similar elements may be used in the matching network. In an exampleembodiment, the matching network is formed from a selected length of pcbtrace that is included in the lamp drive circuit between the amplifier124 and the drive probe 120. These elements are selected both forimpedance matching and to provide a phase shift in the feedback loopthat keeps the amplifier 124 within stable regions of its operation. Aphase shifter 130 may be used to provide additional phase shifting asneeded to keep the amplifier 124 in stable regions.

The amplifier 124 and phase shift in the feedback loop may be designedby looking at the reflection coefficient Γ, which is a measure of thechanging load condition over the various phases of lamp 100 operation,particularly the transition from cold gas at start-up to hot plasma atsteady state. Γ, defined with respect to a reference plane at theamplifier output, is the ratio of the “reflected” electric field E_(in)heading into the amplifier, to the “outgoing” electric field E_(out)traveling out. Being a ratio of fields, Γ is a complex number with amagnitude and phase. One way to depict changing conditions in a systemmay be to use a “polar-chart” plot of Γ's behavior (termed a “loadtrajectory”) on the complex plane. Certain regions of the polar chartmay represent unstable regions of operation for the amplifier 124. Theamplifier 124 and phase shift in the feedback loop may be designed sothe load trajectory does not cross an unstable region. The loadtrajectory can be rotated on the polar chart by changing the phase shiftof the feedback loop (by using the phase shifter 130 and/or adjustingthe length of the circuit loop formed by the lamp drive circuit 106 tothe extent permitted while maintaining the desired impedance matching).The load trajectory can be shifted radially by changing the magnitude(e.g., by using an attenuator).

In example embodiments, radio frequency power may be provided at afrequency in the range of between about 0.1 GHz and about 10 GHz or anyrange subsumed therein. The radio frequency power may be provided todrive probe 120 at or near a resonant frequency for lamp body 102. Thefrequency may be selected based on the dimensions, shape and relativepermittivity of the lamp body 102 to provide resonance in the lamp body102. In example embodiments, the frequency is selected for a fundamentalresonant mode of the lamp body 102, although higher order modes may alsobe used in some embodiments. In other examples, power may be provided ata resonant frequency and/or at one or more frequencies within 1 to 50MHz above or below the resonant frequency or any range subsumed therein.In another example, the power may be provided at one or more frequencieswithin the resonant bandwidth for at least one resonant mode. Theresonant bandwidth is the full frequency width at half maximum of poweron either side of the resonant frequency (on a plot of frequency versuspower for the resonant cavity).

In example embodiments, the amplifier 124 may be operated in multipleoperating modes at different bias conditions to improve starting andthen to improve overall amplifier efficiency during steady stateoperation. For example, the amplifier may be biased to operate in ClassA/B mode to provide better dynamic range during startup and in Class Cmode during steady state operation to provide more efficiency. Theamplifier 124 may also have a gain control that can be used to adjustthe gain of the amplifier 124. The Amplifier 124 may further includeeither a plurality of gain stages or a single stage.

In various examples, the feedback probe 122 is coupled to the input ofthe amplifier 124 through an attenuator 128 and phase shifter 130. Theattenuator 128 is used to adjust the power of the feedback signal to anappropriate level for input to the phase shifter 130. In someembodiments, a second attenuator may be used between the phase shifter130 and the amplifier 124 to adjust the power of the signal to anappropriate level for amplification by the amplifier 124. In someexample embodiments, the attenuator(s) may be variable attenuatorscontrolled by the control electronics 132. In other embodiments, theattenuators may be set to a fixed value. In some example embodiments,the lamp drive circuit 105 may not include an attenuator. In an exampleembodiment, the phase shifter 130 may be a voltage-controlled phaseshifter controlled by the control electronics 132.

The feedback loop may automatically oscillate at a frequency based onthe load conditions and phase of the feedback signal. This feedback loopmay be used to maintain a resonant condition in the lamp body 102 eventhough the load conditions change as the plasma is ignited and thetemperature of the lamp changes. If the phase is such that constructiveinterference occurs for waves of a particular frequency circulatingthrough the loop, and if the total response of the loop (including theamplifier 124, lamp 100, and all connecting elements) at that frequencyis such that the wave is amplified rather than attenuated aftertraversing the loop, the loop may oscillate at that frequency. Whether aparticular setting of the phase-shifter 128 induces constructive ordestructive feedback depends on frequency. The phase-shifter 128 may beused to finely tune the frequency of oscillation within the rangesupported by the lamp's frequency response. In doing so, it may alsoeffectively tune how well RF power is coupled into the lamp 100 becausepower absorption is frequency-dependent. Thus, the phase-shifter 128 mayprovide fast, finely-tunable control of the lamp output intensity. Bothtuning and detuning may be useful. For example: tuning can be used tomaximize intensity as component aging changes the overall loop phase;detuning can be used to control lamp dimming. In some exampleembodiments, the phase selected for steady state operation may beslightly out of resonance, so maximum brightness is not achieved. Thismay be used to leave room for the brightness to be increased and/ordecreased by control electronics 132.

In FIG. 1A, control electronics 132 is connected to attenuator 128,phase shifter 130 and amplifier 124. The control electronics 132 providesignals to adjust the level of attenuation provided by the attenuator128, phase of phase shifter 130, the class in which the amplifier 124operates (e.g., Class A/B, Class B or Class C mode) and/or the gain ofthe amplifier 124 to control the power provided to the lamp body 102. Inone example, the amplifier 124 has three stages, a pre-driver stage, adriver stage and an output stage, and the control electronics 132provides a separate signal to each stage (drain voltage for thepre-driver stage and gate bias voltage of the driver stage and theoutput stage). The drain voltage of the pre-driver stage can be adjustedto adjust the gain of the amplifier. The gate bias of the driver stagecan be used to turn on or turn off the amplifier. The gate bias of theoutput stage can be used to choose the operating mode of the amplifier124 (e.g., Class A/B, Class B or Class C). Control electronics 132 canrange from a simple analog feedback circuit to amicroprocessor/microcontroller with embedded software or firmware thatcontrols the operation of the lamp drive circuit 106. The controlelectronics 132 may include a lookup table or other memory that containscontrol parameters (e.g., amount of phase shift or amplifier gain) to beused when certain operating conditions are detected. In exampleembodiments, feedback information regarding the lamp's 100 light outputintensity is provided either directly by an optical sensor 134, e.g., asilicon photodiode sensitive in the visible wavelengths, or indirectlyby an RF power sensor 136, e.g., a rectifier. The RF power sensor 136may be used to determine forward power, reflected power or net power atthe drive probe 120 to determine the operating status of the lamp. Adirectional coupler may be used to tap a small portion of the power andfeed it to the RF power sensor 136. An RF power sensor 136 may also becoupled to the lamp drive circuit 106 at the feedback probe 122 todetect transmitted power for this purpose. In some embodiments, thecontrol electronics 132 may adjust the phase shifter 130 on an ongoingbasis to automatically maintain desired operating conditions.

High frequency simulation software may be used to help select thematerials and shape of the lamp body 102 and electrically conductivecoating 108 to achieve desired resonant frequencies and field intensitydistribution in the lamp body 102. Simulations may be performed usingsoftware tools such as HFSS, available from Ansoft, Inc. of Pittsburgh,Pa., and FEMLAB, available from COMSOL, Inc. of Burlington, Mass. todetermine the desired shape of the lamp body 102, resonant frequenciesand field intensity distribution. The desired properties may then befine-tuned empirically.

While a variety of materials, shapes and frequencies may be used, oneexample embodiment includes a lamp body 102 designed to operate in afundamental TM resonant mode at a frequency of about 880 MHz (althoughthe resonant frequency changes as lamp operating conditions change asdescribed further below). In this example, the lamp has an alumina lampbody 102 with a relative permittivity of 9.2. The lamp body 102 has acylindrical outer surface as shown in FIG. 1B with a recess 118 formedin the bottom surface. In an alternative embodiment shown in FIG. 1C,the lamp body 102 may have a rectangular outer surface. The outerdiameter D1 of the lamp body 102 in FIG. 1B is about 40.75 mm and thediameter D2 of the recess 118 is about 8 mm. The lamp body 102 has aheight H1 of about 17 mm. A narrow region 112 forms a shelf over therecess 118. The thickness H2 of the narrow region 112 is about 2 mm. Asshown in FIG. 1A, in this region of the lamp body 102 the electricallyconductive surfaces on the lamp body 102 are only separated by the thinregion 112 of the shelf. This results in higher capacitance in thisregion of the lamp body 102 and higher electric field intensities. Thisshape has been found to support a lower resonant frequency than a solidcylindrical body having the same overall diameter D1 and height H1 or asolid rectangular body having the same overall width and height.

In this example, a hole 110 is formed in the thin region 112. The holehas a diameter of about 5.5 mm and the bulb 104 has an outer diameter ofabout 5 mm. The shelf formed by the thin region 112 extends radiallyfrom the edge of the hole 110 by a distance D3 of about 1.25 mm. Aluminapowder is packed between the bulb 104 and the lamp body 102 and forms alayer having a thickness D5 of about ¼ mm. The bulb 104 has an outerlength of about 15 mm and an interior length of about 9 mm. The interiordiameter at the center is about 2 mm and the side walls have a thicknessof about 1.5 mm. The bulb 104 protrudes from the front surface of thelamp body 102 by about 4.7 mm. The bulb 104 has a high pressure fill ofArgon, Kr₈₅, Mercury and Indium Bromide as described above. At pressuresabove 400 Torr, a sparker or other ignition aid may be used for initialignition. Aging of the bulb 104 may facilitate fill breakdown, and thefill may ignite without a separate ignition aid after burn-in of about72 hours.

In this example, the drive probe 120 is about 15 mm long with a diameterof about 2 mm. The drive probe 120 is about 7 mm from the central axisof the lamp body 102 and a distance D4 of about 3 mm from theelectrically conductive material 108 on the inside surface of recess118. The relatively short distance from the drive probe 120 to the bulb104 may enhance coupling of power. In one example, a 15 mm hole isdrilled for the feedback probe 122 to allow the length and coupling tobe adjusted. The unused portion of the hole may be filled with PTFE(Teflon) or another material. In this example, the feedback probe 122has a length of about 3 mm and a diameter of about 2 mm. In anotherembodiment where the length of the hole matches the length of thefeedback probe 122, the length of the feedback probe 122 is about 1.5mm.

In this example, the lamp drive circuit 106 includes an attenuator 128,phase shifter 130, amplifier 124, matching network 126 and controlelectronics 132 such as a microprocessor or microcontroller thatcontrols the drive circuit. A sensor 134 may detect the intensity oflight emitted by the bulb 104 and provide this information to thecontrol electronics 132 to control the drive circuit 106. In analternative embodiment, an RF power sensor 136 may be used to detectforward, reflected or net power to be used by the control electronics tocontrol the drive circuit 106.

The power to the lamp body 102 may be controlled to provide a desiredstartup sequence for igniting the plasma. As the plasma ignites andheats up during the startup process, the impedance and operatingconditions of the lamp change. In order to provide for efficient powercoupling during steady state operation of the lamp 102, in an exampleembodiment, the lamp drive circuit 106 is impedance matched to thesteady state load of the lamp body 102, bulb 104 and plasma after theplasma is ignited and reaches steady state operating conditions. Thismay allow power to be critically coupled from the drive circuit 106 tothe lamp body 102 during steady state operation. However, the power fromthe drive circuit 106 may be over coupled to the lamp body 102 atstartup. As a result, much of the power from the drive circuit 106 isreflected when the lamp 100 is initially turned on. For example, theamplifier 124 may provide about 170 watts of forward power, but morethan half of this power may be reflected at startup. The net power tothe lamp 100 may be only between about 40-80 watts when the power isinitially turned on, and the rest may be reflected.

In one example startup procedure, the lamp 100 starts at a frequency ofabout 895 MHz and the Argon ignites almost immediately. Upon ignition,the frequency drops to about 880 Mhz due to the change in impedance fromthe ignition of the Argon. The Mercury then vaporizes and heats up. TheIndium Bromide also vaporizes and light is emitted at full brightness.When this light is detected by the sensor 134, the phase shifter 130 isadjusted to accommodate for the change in frequency due to the change inthe impedance of the plasma. With the appropriate phase shift, thefeedback loop adjusts the frequency to about 885 MHz. In an exampleembodiment, when this startup process is used with a high pressure fillas described above, the startup process from power on to fullvaporization of the fill may be completed in about 5-10 seconds or less.As a result, full brightness may be achieved very rapidly.

As the plasma continues to heat up, the impedance continues to changeand the frequency continues to drop until the lamp reaches steady stateoperating conditions. As the frequency changes, the phase of the phaseshifter 130 may continue to be adjusted to match the changes infrequency. In an example startup procedure, the frequency ramps down toa steady state operating frequency of about 877 MHz. This ramp may takeseveral minutes. In order to avoid a drop in brightness, the controlelectronics 132 adjusts the phase of the phase shifter 130 in stages tomatch the ramp. A lookup table in the control electronics 132 may beused to store a sequence of parameters indicating the amount of phaseshift to be used by the control electronics 132 during the ramp. In oneexample, the voltage to be applied to the phase shifter 130 is stored inthe lookup table for startup (ignition), full brightness of the plasma(light mode) and steady state after the lamp 100 is heated (run mode).The control electronics 132 may use these parameters to shift the phasein increments between full vaporization of the fill and completion ofheat up. In one example, the lookup table may linearly interpolatebetween the desired phase at full vaporization (light mode) when thefrequency is about 885 MHz and the desired phase at the end of heat up(run mode) when the frequency is about 877 MHz. In one example, firmwarein the control electronics 132 linearly interpolates sixteen values forthe phase voltage that are applied in equal increments over a period ofabout 5 minutes as the lamp 100 ramps from light mode to run mode. Thephase adjustments and ramp may be determined empirically and programmedinto the lookup table based on the operating conditions of theparticular lamp. In an alternative embodiment, the control electronics132 may automatically shift the phase periodically to determine whethera change in one direction or another results in more efficient powercoupling and/or higher brightness (based on feedback from an opticalsensor or rf power sensor in the drive circuit). This periodic phaseshift can be performed very rapidly, so an observer does not notice anyvisible change in the light output intensity.

The phase of the phase shifter 130 and/or gain of the amplifier 124 mayalso be adjusted after startup to change the operating conditions of thelamp 100. For example, the power input to the plasma in the bulb 104 maybe modulated to modulate the intensity of light emitted by the plasma.This can be used for brightness adjustment or to modulate the light toadjust for video effects in a projection display. For example, aprojection display system may use a microdisplay that controls intensityof the projected image using pulse-width modulation (PWM). PWM achievesproportional modulation of the intensity of any particular pixel bycontrolling, for each displayed frame, the fraction of time spent ineither the “ON” or “OFF” state. By reducing the brightness of the lampduring dark frames of video, a larger range of PWM values may be used todistinguish shades within the frame of video. The brightness of the lampmay also be modulated during particular color segments of a color wheelfor color balancing or to compensate for green snow effect in darkscenes by reducing the brightness of the lamp 100 during the greensegment of the color wheel.

In another example, the phase shifter 130 can be modulated to spread thepower provided by the lamp circuit 106 over a larger bandwidth. This canreduce Electromagnetic Interference (EMI) at any one frequency andthereby help with compliance with FCC regulations regarding EMI. Inexample embodiments, the degree of spectral spreading may be from 5-30%or any range subsumed therein. In one example, the control electronics132 may include circuitry to generate a saw tooth voltage signal and sumit with the control voltage signal to be applied to the phase shifter130. In another example, the control electronics 132 may include amicrocontroller that generates a PWM signal that is passed through anexternal low-pass filter to generate a modulated control voltage signalto be applied to the phase shifter 130. In example embodiments, themodulation of the phase shifter 130 can be provided at a level that iseffective in reducing EMI without any significant impact on the plasmain the bulb 104.

In example embodiments, the amplifier 124 may also be operated atdifferent bias conditions during different modes of operation for thelamp 100. The bias condition of the amplifier 124 may impact DC-RFefficiency. For example, an amplifier biased to operate in Class C modeis more efficient than an amplifier biased to operate in Class B mode,which in turn is more efficient than an amplifier biased to operate inClass A/B mode. However, an amplifier biased to operate in Class A/Bmode has a better dynamic range than an amplifier biased to operate inClass B mode, which in turn has better dynamic range than an amplifierbiased to operate in Class C mode.

In one example, when the lamp is first turned on, amplifier 124 isbiased in a Class A/B mode. Class A/B provides better dynamic range andmore gain to allow amplifier 124 to ignite the plasma and to follow theresonant frequency of the lamp 100 as it adjusts during startup. Oncethe lamp 100 reaches full brightness, amplifier bias is removed whichputs amplifier 124 into a Class C mode. This may provide improvedefficiency. However, the dynamic range in Class C mode may not besufficient when the brightness of the lamp 100 is modulated below acertain level (e.g., less than 70% of full brightness). When thebrightness is lowered below the threshold, the amplifier 124 may bechanged back to Class A/B mode. Alternatively, Class B mode may be usedin some embodiments.

The above dimensions, shape, materials and operating parameters areexamples only and other embodiments may use different dimensions, shape,materials and operating parameters.

FIG. 1D shows a cross-section and schematic view of a plasma lamp 200according to an example embodiment. The plasma lamp 200 may have a lampbody 202 formed from one or more solid dielectric materials and a bulb204 positioned adjacent to the lamp body 202. The bulb 204 contains afill that is capable of forming a light emitting plasma. A lamp drivecircuit (e.g., a lamp drive circuit 206 shown by way of example in FIG.1H) couples radio frequency (RF) power into the lamp body 202 which, inturn, is coupled into the fill in the bulb 204 to form the lightemitting plasma. In example embodiments, the lamp body 202 forms astructure that contains and guides the radio frequency power.

In the plasma lamp 200 the bulb 204 is shown to be positioned ororientated so that a length of a plasma arc 268 generally faces a lampopening 210 (as opposed to facing side walls 270) to increase an amountof collectable light emitted from the plasma arc 268 in a given etendue.Since the length of plasma arc 268 orients in a direction of an appliedelectric field, the lamp body 202 and the coupled RF power may providean electric field 273 that is aligned or substantially parallel to thelength of the bulb 204 and a front or upper surface 214 of the lamp body202. Thus, in an example embodiment, the length of the plasma arc 268may be substantially (if not completely) visible from outside the lampbody 202. In example embodiments, collection optics 274 may be in theline of sight of the full length of the bulb 204 and plasma arc 268. Inother examples, about 40%-100%, or any range subsumed therein, of theplasma arc 268 may be visible to the collection optics 274 in front ofthe lamp 200. Accordingly, the amount of light emitted from the bulb 204and received by the collection optics 274 may be enhanced. In exampleembodiments, a substantial amount of light may be emitted out of thelamp 200 from the plasma arc 268 through a front side wall of the lamp200 without any internal reflection.

As described herein, the lamp body 202 is configured to realize thenecessary resonator structure such that the light emission of the lamp200 is enabled while satisfying Maxwell's equations.

In FIG. 1D, the lamp 200 is shown to include a lamp body 202 including asolid dielectric body and an electrically conductive coating 208 whichextends to the front or upper surface 214. The lamp 200 is also shown toinclude dipole arms 276 and conductive elements 278, 280 (e.g.,metallized cylindrical holes bored into the body 202) to concentrate theelectric field present in the lamp body 202. The dipole arms 276 maythus define an internal dipole. In an example embodiment, a resonantfrequency applied to a lamp body 202 without dipole arms 276 andconductive elements 278, 280 would result in a high electric field atthe center of the solid dielectric lamp body 202. This effect wouldresult from the intrinsic resonant frequency response of the lamp body202 due to its shape, dimensions and relative permittivity. However, inthe example embodiment of FIG. 1D, the shape of the standing waveforminside the lamp body 202 is substantially modified by the presence ofthe dipole arms 276 and conductive elements 278, 280 and the electricfield maxima is brought out to end portions 282, 284 of the bulb 204using the internal dipole structure. This results in the electric filed273 near the upper surface 214 of the lamp 200 that is substantiallyparallel to the length of the bulb 204. In some example embodiments,this electric field is also substantially parallel to a drive probe 220and feedback probe 222 (see FIG. 1H below).

That the plasma arc 268 in lamp 200 is oriented such that it presents along side to the lamp exit aperture or opening 210 may provide variouseffects. The basic physical difference relative to an “end-facing”orientation of the plasma arc 268 is that much of the light can exit thelamp 200 without suffering multiple reflections within the lamp body202. Therefore, a specular reflector may show a significant improvementin light collection performance over a diffuse reflector that may beutilized in a lamp with an end facing orientation. An example embodimentof a specular reflector geometry that may be used in some embodiments isa parabolic line reflector, positioned such that the plasma arc lies inthe focal-line of the reflector.

Another difference may lie in that the side wall of the bulb 204 can berelatively thick, without unduly inhibiting light collectionperformance. Again, this is because the geometry of the plasma arc 268with respect to the lamp opening 210 is such that the most of the lightemanating from the plasma arc 268 will traverse thicker walls at anglescloser to normal, and will traverse them only once or twice (or at leasta reduced number of times). In example embodiments, the side wall of thebulb 204 may have a thickness in the range of about 1 mm to 10 mm or anyrange subsumed therein. In one example, a wall thickness greater thanthe interior diameter or width of the bulb 204 may be used (e.g., 2-4 mmin some examples). Thicker walls may allow higher power to be coupled tothe bulb 204 without damaging the wall of the bulb 204. This is anexample only and other embodiments may use other bulbs. It will beappreciated that the bulb 204 is not restricted to a circularcylindrical shape and may have more than one side wall.

FIGS. 1E-1G show more detailed diagrams of the example plasma lamp 200shown in FIG. 1D. The lamp 200 is shown in exploded view and includesthe electrically conductive coating 208 (see FIG. 1G) provided on aninternal solid dielectric 286 defining the lamp body 202. The oblongbulb 204 and surrounding interface material 216 (see FIG. 1E) are alsoshown. Power may be fed into the lamp 200 with an electric monopoleprobe closely received within a drive probe passage 288. The twoopposing conductive elements 278, 280 of FIG. 1D may be formedelectrically by the metallization of the bore 238 (see FIG. 1G), whichextend toward the center of the lamp body 202 (see also FIG. 1D) toconcentrate the electric field, and build up a high voltage to energizethe lamp 200. The dipole arms 276 connected to the conductive elements278, 280 by conductive surfaces may transfer the voltage out towards thebulb 204. The cup-shaped terminations or end portions 240 on the dipolearms 276 partially enclose the bulb 204. A feedback probe passage 242 isshown in the lamp body 202 and is to snugly receive a feedback probe 222that connects to a drive circuit (e.g. a lamp drive circuit 206 shown byway of example in FIG. 1H). In an example embodiment the interfacematerial 216 may be selected so as to act as a specular reflector toreflect light emitted by the plasma arc 268.

Referring to FIG. 1F, the lamp body 202 is shown to include three bodyportions 244, 246 and 248. The body portions 244 and 248 are mirrorimages of each other and may each have a thickness 250 of about 11.2 mm,a height 252 of about 25.4 mm and width 254 of about 25.4 mm. The innerportion 246 may have a thickness 255 of about 3 mm. The lamp opening 210in the upper surface 214 may be partly circular cylindrical in shapehaving a diameter 256 of about 7 mm and have a bulbous end portions witha radius 258 of about 3.5 mm. The drive probe passage 288 and thefeedback probe passage 242 may have a diameter 260 of about 1.32 mm. Arecess 262 with a diameter 264 is provided in the body portion 248. Thebores 238 of the conductive elements 278, 280 may have a diameter 266 ofabout 7 mm.

An example analysis of the lamp 200 using 3-D electromagnetic simulationbased on the finite-integral-time-domain (FITD) method is describedbelow with reference to FIGS. 3-5. The electric (E) field (see FIG. 5),the magnetic (H) field (see FIG. 4), and the power flow (which is thevectoral product of the E and H fields—see FIG. 3), are separatelydisplayed for insight, although they are simply three aspects of thetotal electromagnetic behavior of the lamp 200. In the exampleembodiment simulated in the three figures, a drive probe 270 couplespower into the lamp body 202 and a feedback probe 272 is placed on thesame side of the body 202 as the drive probe 270. This is an alternativeembodiment representing only a superficial difference from theconfiguration of drive and feedback probes for use in the exampleembodiment shown in FIGS. 1E-1G.

In the example lamp drive circuit 206 shown in FIG. 1H, the controlelectronics 232 is connected to the attenuator 228, the phase shifterphase shifter 230 and the amplifier 224. The control electronics 232provide signals to adjust the level of attenuation provided by theattenuator 228, the phase of phase shifter 230, the class in which theamplifier 224 operates (e.g., Class A/B, Class B or Class C mode) and/orthe gain of the amplifier 224 to control the power provided to the lampbody 202. In one example, the amplifier 224 has three stages, apre-driver stage, a driver stage and an output stage, and the controlelectronics 232 provides a separate signal to each stage (drain voltagefor the pre-driver stage and gate bias voltage of the driver stage andthe output stage). The drain voltage of the pre-driver stage can beadjusted to adjust the gain of the amplifier 224. The gate bias of thedriver stage can be used to turn on or turn off the amplifier 224. Thegate bias of the output stage can be used to choose the operating modeof the amplifier 224 (e.g., Class A/B, Class B or Class C). The controlelectronics 232 can range from a simple analog feedback circuit to amicroprocessor/microcontroller with embedded software or firmware thatcontrols the operation of the lamp drive circuit 206. The controlelectronics 232 may include a lookup table or other memory that containscontrol parameters (e.g., amount of phase shift or amplifier gain) to beused when certain operating conditions are detected. In exampleembodiments, feedback information regarding the lamp's light outputintensity is provided either directly by the optical sensor 234, e.g., asilicon photodiode sensitive in the visible wavelengths, or indirectlyby the RF power sensor 236, e.g., a rectifier. The RF power sensor 236may be used to determine forward power, reflected power or net power atthe drive probe 270 to determine the operating status of the lamp 200.Matching network 226 may be designed to also include a directionalcoupler section, which may be used to tap a small portion of the powerand feed it to the RF power sensor 236. The RF power sensor 236 may alsobe coupled to the lamp drive circuit 206 at the feedback probe 272 todetect transmitted power for this purpose. In some example embodiments,the control electronics 232 may adjust the phase shifter 230 on anongoing basis to automatically maintain desired operating conditions.

As described above, the phase of the phase shifter 230 and/or gain ofthe amplifier 224 may also be adjusted after startup to change theoperating conditions of the lamp 200, which can be used for brightnessadjustment or to modulate the light to adjust for video effects in aprojection display.

As described above, the phase shifter 230 can be modulated to spread thepower provided by the lamp circuit 206 over a larger bandwidth, whichmay reduce EMI at any one frequency and thereby help with compliancewith FCC regulations regarding EMI

As described above, the amplifier 224 may be operated at different biasconditions during different modes of operation for the lamp 200, whichmay impact DC-RF efficiency.

Further non-limiting example embodiments are shown in FIGS. 1I and 1J.However, it should be noted that these embodiments are shown merely byway of example and not limitation.

FIG. 1I shows a cross-section and schematic view of a plasma lamp 200,according to an example embodiment, in which a bulb 204 of the lamp 200is orientated to enhance an amount of collectable light into a givenetendue. The lamp 200 is shown to include a lamp body 202 including asolid dielectric resonator, and an electrically conductive coating 208.In this example, an artificial magnetic wall 294 is used to modifyorientation of the electric field. An ideal magnetic wall, made from anideal magnetic conductor which does not exist in nature, would permit anelectric field to point parallel to its surface, which is the desiredconfiguration for this example embodiment. Approximations to an idealmagnetic conductor exist in the form of a planar surface patterned withperiodic regions of varying conductivity. Such a structure, belonging tothe family of periodically-patterned structures collectively known asPhotonic Bandgap devices, permit among other things parallel attachedelectric fields when the relationship between the wavelength of thefield and the periodicity of the structure is correctly designed. (see:F R Yang, K P Ma, Y Qian, T Itoh, A novel TEM waveguide using uniplanarcompact photonic-bandgap (UC-PBG) structure, IEEE Transactions onMicrowave Theory and Techniques, November 1999, v47 #11, p 2092-8),which is hereby incorporated herein by reference in its entirety). Forexample, a unipolar compact photonic bandgap (UC-PBG) structure of thetype described in this article may be used on a surface of the lamp body202 in example embodiments to provide a magnetic boundary condition. Arepeating unit used in an example photonic bandgap lattice has squarepads and narrow lines with insets, as shown in FIG. 1I. The gaps betweenadjacent units provide capacitance. The branches and insets provideinductance. This forms a distributed LC circuit and has a particularfrequency response. This structure can be tuned to provide an equivalentmagnetic surface at particular frequencies, and can be scaled fordifferent frequency bands. As a result, it is believed that a photonicbandgap lattice structure may be used in example embodiments to providea magnetic boundary condition and adjust the orientation of the electricfield to be substantially parallel to the length of the bulb 204adjacent to a front surface of the lamp body 202. This is an exampleonly and other structures may be used to provide a magnetic boundarycondition in other embodiments.

FIG. 1J shows a cross-section and schematic view of a further exampleembodiment of a plasma lamp 200, in which a bulb 204 of the lamp 200 isorientated to enhance an amount of collectable light into a givenetendue. The lamp 200 is shown to include a lamp body 202 including asolid dielectric resonator and an electrically conductive coating 208which extends to a front or upper surface 214. The lamp body 202 isprovided with the electrically conductive coating 208 such that there isa partial gap 296 in the electrically conductive coating 208 along amidplane of the bulb 204. An internal cavity or chamber 298 extends intothe lamp body 202. The conductive coating 208 may also extend into thecavity 298. In this example embodiment, end portions 282, 284 of thebulb 204 extend below the electrically conductive coating 208 on theupper surface 214 of the lamp body 202. This lamp 200 may operates in amanner similar to a vane resonator with a solid dielectric body.

FIG. 1K shows a cross-sectional view of a lamp 201 according to anotherexample embodiment. The lamp 201 is similar to the lamp of FIG. 1Hexcept that it does not have a feedback probe and uses a different powercircuit. The lamp 201 includes a bulb 204, a lamp body 202, conductiveelements 278 and 280, an electrically conductive layer 208, dipole arms276, a drive probe 270 and a sensor 234. As shown in FIG. 1K, a lampdrive circuit 207 is shown to include an oscillator 292 and an amplifier224 (or other source of radio frequency (RF) power) may be used toprovide RF power to the drive probe 270. The drive probe 270 is embeddedin the solid dielectric body of the lamp 201. Control electronics 233controls the frequency and power level provided to the drive probe 270.Control electronics 233 may include a microprocessor or microcontrollerand memory or other circuitry to control the lamp drive circuit 207. Thecontrol electronics 233 may cause power to be provided at a firstfrequency and power level for initial ignition, a second frequency andpower level for startup after initial ignition and a third frequency andpower level when the lamp 201 reaches steady state operation. In someexample embodiments, additional frequencies may be provided to match thechanging conditions of the load during startup and heat up of theplasma. For example, in some embodiments, more than sixteen differentfrequencies may be stored in a lookup table and the lamp 201 may cyclethrough the different frequencies at preset times to match theanticipated changes in the load conditions. In other embodiments, thefrequency may be adjusted based on detected lamp operating conditions.The control electronics 233 may include a lookup table or other memorythat contains control parameters (e.g., frequency settings) to be usedwhen certain operating conditions are detected. In example embodiments,feedback information regarding the lamp's light output intensity isprovided either directly by an optical sensor 234, (e.g., a siliconphotodiode sensitive in the visible wavelengths), or indirectly by an RFpower sensor 236, e.g., a rectifier. The RF power sensor 236 may be usedto determine forward power, reflected power or net power at the driveprobe 270 to determine the operating status of the lamp 201. Adirectional coupler 290 may be used to tap a small portion of the powerand feed it to the RF power sensor 236. In some embodiments, the controlelectronics 233 may adjust the frequency of the oscillator 292 on anongoing basis to automatically maintain desired operating conditions.For example, reflected power may be minimized in some embodiments andthe control electronics may rapidly toggle the frequency to determinewhether an increase or decrease in frequency will decrease reflectedpower. In other examples, a brightness level may be maintained and thecontrol electronics may rapidly toggle the frequency to determinewhether the frequency should be increased or decreased to adjust forchanges in brightness detected by sensor 234.

The above circuits, dimensions, shapes, materials and operatingparameters are examples only and other embodiments may use differentcircuits, dimensions, shapes, materials and operating parameters.

FIG. 2 shows a plasma lamp 300 similar to embodiments of plasma lampsdescribed above. The plasma lamp 300 includes a conductor 308. In oneexample embodiment, the conductor is formed as a coating that is paintedon, or otherwise applied, although the invention is not so limited.Other conductors such as sheets can be physically attached, etc. Anexample of a coated conductor 308 includes a silver painted coating. Theconductor 308 is shown covering a portion of a dielectric base 302,leaving a gap 310. The gap 310 is shown with a thickness 312. In oneexample embodiment, the gap thickness 312 ranges from 1 to 10 mm.

A bulb 304 is shown located within an opening in the dielectric base302. Similar to embodiments described above, an example of the bulb 304includes a transparent, heat resistant material such as quartz thatincludes a gas, or other material within the bulb 304 for excitationinto a plasma state. As shown in the example of FIG. 2, the bulb 304 isat least partially surrounded by a recess 318. In some configurations,the recess 318 is filled with a dielectric material. One example ofdielectric material for the recess 318 includes alumina powder. Amixture of powder and air may provide thermal insulation to reduce heattransfer to adjacent structures.

In example configurations it may be desirable to reduce the gapthickness 312. Such a reduction may provide improved lighting propertiesof plasma formed within the bulb 304. In one example embodiment,reduction of gap thickness 312 may control a dimension of a plasmaregion within the bulb 304 to tailor the light distribution. Reductionof the gap thickness 312, however, introduces other technical challengesto the configuration.

One technical challenge that is introduced as gap thickness 312 isreduced includes electrical arcing between a first end 314 of theconductor 308 and a second end 316 of the conductor 308. The arcing canoccur across the gap 310 for a number of reasons, including reduction ofthe gap thickness 312. Other factors that contribute to arcing includeincreased voltage, surges in voltage, and geometry of the conductor 308.For example, arcing may be caused by increased electrical potential atcorners of the conductor 308 such as at ends 314 and 316.

Arcing occurs when the media in the region of the gap 310 is ionized ora breakdown voltage of the media is exceeded. For example, air oralumina powder in a portion of the recess 318 adjacent to the gap 310can ionize and an arc may occur. Another example of arcing includesbreakdown of portions of the conductor 308 (e.g., of the silver) andarcing may occur.

In one example embodiment, a dielectric coating 350 is applied over aportion of the conductor 308 and over the gap 310. The dielectriccoating 350 includes material properties that overcome technical hurdlessuch as arcing, and further satisfy other material needs for applicationwithin the plasma lamp 300. In one example embodiment, a breakdownvoltage of the dielectric coating is higher than a breakdown voltage ofair. It is to be noted that the application of a non-conductive coatingmay be provided at any point and over any surface of the lamp 200 wherethere is a possibility of arcing.

An example of a dielectric coating 350 includes a glass coating such assilicon dioxide. Other glasses or mixtures of glasses are also withinthe scope of the example embodiments. In one example embodiment, thedielectric coating 350 is adhered to a surface of the gap 310. Anadhered dielectric coating 350 reduces a presence of air at theinterface between the dielectric coating 350 and the dielectric of thedielectric base 302 in the gap 310. In one example embodiment, thedielectric coating 350 has a higher breakdown voltage than air.

The dielectric coating 350 may be selected so as to be able to withstandtemperatures in excess of 100 degrees Celsius. In an example embodiment,the dielectric coating 350 may experience temperatures in excess of 350degrees Celsius. In an example embodiment, a temperature at the bulb 304may be approximately 800 degrees Celsius.

The dielectric coating 350 may be transparent, include a favorablecoefficient of thermal expansion, hardness, fracture toughness,adhesion, and microstructure. Transparency of a dielectric coating 350may facilitate inspection of components underneath the dielectriccoating 350 such as the conductor 308. A coefficient of thermalexpansion of the dielectric coating 350 may match surroundingcomponents. Due to large temperature variations, in example embodimentsan insufficient match of the coefficient of thermal expansion betweenthe dielectric coating 350 and surrounding components may lead tofailure at the coating interface. Mechanical properties of thedielectric coating 350 such as hardness, fracture toughness, andmicrostructure may provide a robust coating that can withstandmechanical forces during manufacturing. The dielectric coating may beselected to facilitate adhesion. In one example embodiment, thedielectric coating 350 material is chosen to provide good adhesion tothe conductor 308 and the base 302 without any intermediate coatingsthat would add complexity and expense to the manufacturing process.

In an example embodiment, the dielectric coating 350 includes a glassfrit coating. One example of a suitable glass frit includes QM44 fromDuPont Microcircuit Materials. Properties may include a breakdownvoltage of greater than 1000 V/25 μm. In one example of a glass frit,such as QM44, small glass particles are included in an organic orsolvent carrier. The glass frit may be painted, dipped, or otherwiseapplied to a surface such as the surfaces of the conductor 308 and thegap 310. The glass frit may then be fired to remove the carrier andleave behind the glass particles adhered to the desired surfaces. In oneexample, the dielectric coating 350 is formed to a thickness between 3and 30 μm. For example, the dielectric material may be selected so thatit has a breakdown voltage of higher the air (3000 V/mm). When thenon-conductive dielectric material is glass, the breakdown voltage maythus be 40 times greater than air. It is however to be appreciated that,in certain embodiments, the greater the breakdown voltage of thenon-conductive dielectric material, the likelihood of arcing is furtherreduced.

In an example embodiment, this thickness may be sufficient to result ina high breakdown voltage, and thin enough to meet packaging andmanufacturing constraints for the plasma lamp 300. In the exampleembodiment shown in FIG. 2, the dielectric coating 350 covers some, butnot all of the conductor 308 and the base 302. In one exampleembodiment, the dielectric coating 350 is applied by dipping the plasmalamp 300 into the glass frit material. A line above which the plasmalamp 300 was not dipped is visible in the illustration of FIG. 2.

Glass frit coatings formed similar to this process as described mayprovide the properties such as transparency, high breakdown voltage,thermal expansion, etc. The method of application may provide a highhardness coating with a unique microstructure that is resistant tocracking and other mechanical damage. Application of a glass fritcoating as described above may also be convenient for manufacturing. Forexample dipping parts and batch firing are possible using the methodsdescribed above.

It is to be noted that a non-conductive coating with a high breakdownvoltage may be provided at any location of the plasma lamp where arcingcan potentially occur. For example, imperfections in the metal coating308 may also give rise to arcing. Examples of such an imperfection areburs, scratching, or the like in the metal coating that may causearcing. Thus, in an example embodiment, a non-conductive coating (e.g.,a glass coating) may be provided at any one or more locations on anymetal surface where a high potential difference exists so as to at leastreduce the likelihood of arcing. For example, the coating may beprovided where the there is a gap of between 2 and 5 mm between twopoints and a likelihood exists that arcing may take place due to thepotential difference between the two points. The non-conductivedielectric coating may also be applied to the lamps 200 shown in FIGS.1I and 1J where arcing may arise due to high potential differences.

In the example lamp body 202 shown in FIG. 1F, the non-conductivedielectric coating may be provided, at least partially, on exposed outersurfaces of the body portions 144 and 148. In an example embodiment, thenon-conductive dielectric coating may extend from an upper/top surfaceof each body portion 144, 148 into the lamp opening 210 to at leastreduce the likelihood of arcing between the upper/top surface andcup-shaped terminations or end portions 240 (see also FIG. 1D). In anexample embodiment, the non-conductive dielectric material may beprovided on the central portion 246. For example, opposed major surfacesof the portion 246 may be coated with a glass frit to inhibit arcingbetween the dipole arms 276.

Further, in an example embodiment, the non-conductive coating (e.g.,glass frit) may at least partially cover the dipole arms 276 to reducethe likelihood of arcing.

FIG. 3 shows a plasma lamp 400 including similar features to the exampleembodiments described above. The plasma lamp 400 includes a conductor408. The conductor 408 is shown covering a portion of a dielectric base402, leaving a gap. A bulb 404 is shown located within an opening in thedielectric base 402. As shown in the example of FIG. 3, the bulb 404 isat least partially surrounded by a recess 418. In example embodiments,the recess 418 is filled with a dielectric material. One example ofdielectric material for the recess 418 includes alumina powder. Amixture of powder and air may provide thermal insulation to reduce heattransfer to adjacent structures.

A coating 460 is shown covering a portion of the conductor 408. In oneexample embodiment, the coating 408 is conductive, although theinvention is not so limited. It has been found that conductors made ofmaterials such as silver or gold, when exposed to high operatingtemperatures such as 350 to 800 degrees Celsius are more susceptible toreactions with the atmosphere. Tarnish from sulfur is an example problemthat may arise. Because Ag₂S and similar gold sulfides arenon-conductive, as tarnish consumes the conductor 408, it may cease tobe a conductor, thus reducing effectiveness of the lamp 400 or causinglamp failure.

In one example embodiment, the coating 460 includes a barrier layer suchas a passivation layer or other layer that prevents sulfur from reactingwith the conductor 408. Although sulfur tarnish may be of particularconcern, the example is not so limited. Other adverse reactions andbarrier layers to at least reduce, or prevent, them are within the scopeof the example embodiments. In one example embodiment, the coating 460may cover selected areas of the plasma lamp 400 or conductor 408 thatare exposed to high heat. As discussed above, regions adjacent to thebulb 404 as shown in FIG. 3 may be particularly susceptible to tarnishdue to faster chemical reactions at elevated temperatures.

In one example embodiment, the coating 460 includes nickel. Throughtesting, nickel has been found to be effective at preventing tarnish dueto sulfur exposure. Although nickel has the advantage of beingconductive, in one example the coating 460 includes a non-conductivematerial. In an example embodiment, a glass coating such as a glass fritcoating as described above may be used. Combinations of coatings arealso within the scope of the example embodiments. For example, in anexample embodiment, both a nickel coating may be formed over a portionof a conductor, and a glass frit coating may be formed over theconductor and nickel coating. In this way, the conductor may beprotected from tarnish by both the nickel and the glass, while a gap isprotected from problems such as arcing.

FIG. 4 illustrates a method according to an embodiment. As discussedabove, one example method of applying a dielectric coating 250 includescoating at least a portion of the pair of electrodes and the gap with aglass frit slurry. As illustrated, in one example method, the glass fritslurry is heated to remove a carrier material and adhere the glass fritto a surface of the pair of electrodes and a surface of the gap

FIG. 5 illustrates one example method of forming a barrier layer. Themethod includes forming a pair of conductors over a dielectric base witha gap between the pair of conductors, and placing a bulb into an openingwithin the dielectric base, adjacent to the gap. The method furtherincludes forming a barrier layer over at least a portion of at least oneof the pair of conductors. In one example method, as discussed above,the barrier layer includes nickel. In an example embodiment, the barrierlayer includes glass frit. Although nickel and glass frit are used asexamples, the example embodiments are not so limited. One of ordinaryskill in the art, having the benefit of the present disclosure willrecognize that other barrier layer materials and methods of applicationare within the scope of the invention.

1. An electrodeless plasma lamp comprising: a lamp body comprising adielectric material, the lamp body having an outer conductor forming aresonant structure; a radio RF power source configured to provide RFpower to the resonant structure; a bulb adjacent to the lamp body, thebulb containing a fill that forms a plasma when the RF power is coupledto the fill from the lamp body; at least one inner conductor having anend proximate the bulb, the end being spaced from the outer conductor toprovide a gap in the conductive coating; and a dielectric coating overat least the end of the inner conductor to inhibit arcing at the end,the dielectric coating having a higher breakdown voltage than air. 2.The electrodeless plasma lamp of claim 1, wherein the inner conductor isa conductive coating on an inner surface of the lamp body.
 3. Theelectrodeless plasma lamp of claim 1, wherein the lamp body includes anopening extending inwardly from an outer surface of the lamp body toprovide an internal cavity in the lamp body to at least partiallyreceive the bulb.
 4. The electrodeless plasma lamp of claim 3, whereinthe end of the inner conductor is located within the cavity.
 5. Theelectrodeless plasma lamp of claim 1, wherein an end portion of theinner conductor extends parallel to an upper surface of the lamp body.6. The electrodeless plasma lamp of claim 1, comprising a first innerconductor and second inner conductor, the first inner conductor having afirst end and the second inner conductor having a second end, whereinthe dielectric coating covers at least the first and the second ends. 7.The electrodeless plasma lamp of claim 6, wherein the first and secondends form part of dipole arms to couple power from the lamp body intothe bulb.
 8. The electrodeless plasma lamp of claim 1, wherein thedielectric coating is further provided over at least a portion of theouter conductor.
 9. The electrodeless plasma lamp of claim 8, whereinthe lamp body includes an opening extending inwardly from an outersurface of the lamp body to provide an internal cavity in the lamp bodyto at least partially receive the bulb; and the dielectric coating isprovided over at least a portion of the outer conductor and extends atleast partially into the internal cavity.
 10. The electrodeless plasmalamp of claim 1, wherein the dielectric coating has a thickness ofbetween 3 and 30 μm.
 11. The electrodeless plasma lamp of claim 1,wherein the dielectric coating includes a breakdown voltage greater thanor equal to 40 V/μm.
 12. The electrodeless plasma lamp of claim 1,wherein the dielectric coating maintains its material properties attemperatures up to 350 degrees Celsius.
 13. The electrodeless plasmalamp of claim 1, wherein the dielectric coating includes a coefficientof thermal expansion that substantially matches the coefficient ofthermal expansion of the lamp body.
 14. The electrodeless plasma lamp ofclaim 1, wherein the dielectric coating includes a glass frit coating.15. The electrodeless plasma lamp of claim 14, wherein the glass fritcoating is a silicon dioxide glass frit coating.
 16. The electrodelessplasma lamp of claim 1, further comprising a barrier layer provided overat least a portion of the outer conductor.
 17. The electrodeless plasmalamp of claim 16, wherein the barrier layer is formed from a conductivematerial.
 18. The electrodeless plasma lamp of claim 16, wherein thebarrier layer is formed from a material that prevents sulfur attack onthe conductive coating.
 19. The electrodeless plasma lamp of claim 16,wherein the barrier layer includes nickel.
 20. The electrodeless plasmalamp of claim 1, wherein a shortest distance between an end of the bulband a point on a RF feed traverses at least portion of the internalconductor.
 21. The electrodeless plasma lamp of claim 20, wherein thebulb has an exposed end from which light exits the plasma lamp, and aconcealed end, the shortest distance being between the concealed end ofthe bulb and the RF feed.
 22. The electrodeless plasma lamp of claim 1,wherein the internal conductor is electrically coupled to the outerconductor.